[llvm] [LV] Add support for partial alias masking with tail folding (PR #182457)

[Gaëtan Bossu via llvm-commits]

================
@@ -1382,6 +1388,48 @@ class LoopVectorizationCostModel {
     return getTailFoldingStyle() != TailFoldingStyle::None;
   }
 
+  void tryToEnablePartialAliasMasking() {
+    assert(!IsPartialAliasMaskingEnabled && "Alias masking already enabled!");
+    assert(foldTailByMasking() && "Expected tail folding to be enabled!");
+    assert(!foldTailWithEVL() &&
+           "Did not expect to enable alias masking with EVL!");
+    // Note: FixedOrderRecurrences are not supported yet as we cannot handle
+    // the required `splice.right` with the alias-mask.
+    if (!ForcePartialAliasingVectorization ||
+        !Legal->getFixedOrderRecurrences().empty())
+      return;
+
+    const RuntimePointerChecking *Checks = Legal->getRuntimePointerChecking();
+    if (!Checks)
+      return;
+
+    auto DiffChecks = Checks->getDiffChecks();
+    if (!DiffChecks || DiffChecks->empty())
+      return;
+
+    for (BasicBlock *BB : TheLoop->blocks()) {
+      for (Instruction &I : *BB) {
+        if (!isa<LoadInst, StoreInst>(I))
----------------
gbossu wrote:

I guess that we currently only expect loads and stores to touch memory. I assume this is guarded somewhere in legality checks. But I think it's worth adding a bit of future-proofing here: either asserting that `!I.mayReadOrWriteMemory()` when it's not a ld/st or disabling alias masking when that is the case.

https://github.com/llvm/llvm-project/pull/182457